The process of walking is a sequence of steps, each of which is characterized by certain landmark events. These landmark events are often referred to as “gait events.” The five gait events most often used in the analysis of gait are: weight acceptance (“WA”), mid-stance (“MSt”), terminal stance (“TSt”), pre-swing (“PSw”), and initial swing (ISw”). These events are described in detail by Perry, “Gait Analysis: Normal and Pathological Function,” Delmar Thomson Learning, Clifton Park, N.Y., 1992, the contents of which are incorporated herein by reference in their entirety.
The time between gait events may be short, as in a brisk walk, or it may be long, as in a leisurely stroll. However, regardless of the time interval between them, these gait events are expected to occur in the same order. Moreover, the time intervals between gait events are expected to be related. For example, if the interval between pre-swing and initial swing is short, then the interval between terminal stance and pre-swing should also be short, otherwise the gait will degenerate into a sequence of lurches.
When walking, the legs move into positions that are appropriate for each gait event. To carry this out requires cooperation between muscles, which move the legs, and nerve impulses, which trigger the muscles into contracting at appropriate times. In persons afflicted with cerebral palsy, the muscles are available, but the nerve signals either do not arrive, or they do not arrive at the correct times.